Ukraine's military and pro-Russian rebels are accusing each other of shelling residential districts in and near the war-shattered rebel-held city of Donetsk, after police on Sunday reported at least four civilians killed in the past 24 hours.

Ukrainian military officials say they observed rebels firing missiles toward the city center and then redirecting fire toward Ukrainian troops dug in nearby. Spokesman Serhiy Halushko also said the army had intercepted rebel communications suggesting separatists were planning to shell the city.

Senior rebel official Eduard Basurin told Reuters he had no explanation for the shelling, which analysts say was the heaviest on the city center since a tenuous cease-fire was announced in February. "Do you believe we can shell ourselves?" he asked.

The United Nations says more than 6,400 people have died in the conflict since rebels launched a separatist rebellion in Ukraine's mostly Russian-speaking east in April 2014.

Monitors from the Organization for Security and Cooperation in Europe say neither side has fully complied with terms of the February truce, which called for both sides to withdraw heavy artillery from the frontline.